The conventional feedback air-fuel ratio regulator of this type is so designed as to transmit a signal that changes with a certain time constant when the air-fuel ratio sensor has detected deviation of the air-fuel ratio from a preset theoretical value. Then, the amount of fuel to be injected is regulated so that the deviated air-fuel ratio is brought back again to the theoretical level by means of a signal transmitted from the electronic fuel injection controlling circuit into which the aforesaid signal and other compensating signals, such as one indicating the amount of sucked gas and one from the distributor for the spark plugs, are inputted.
Then, while fuel is injected into the intake air, the air-fuel ratio sensor is installed in a rather limited portion of its exhaust section. This entails a time-lag between the injection of fuel and the detection of air-fuel ratio that is equivalent to the sum of the time during which the sucked mixture is held within the cylinder and the time required for it to pass through the intake and exhaust passages. As a consequence, fuel has to be overinjected by such amount that corresponds to this time-lag, in excess of the amount required for attaining the theoretical air-fuel ratio, which is undesirable for the cleaning of exhaust gas. Said overinjection can be reduced by making the value of said time constant large. Then, however, it will take a considerable time for air-fuel ratio to return to its theoretical value subsequent to sudden change, impairing the response of the engine or the drive performance of the automobile.
The primary object of this invention is to provide a feedback air-fuel ratio regulator of the type which assures both a high drive performance and a good exhaust gas cleaning function by rapidly changing the amount of fuel injection with a small time constant until air-fuel ratio reaches the theoretical value, and reducing the amount of oversupply within the time-lag by automatically increasing said time constant after said theoretical air-fuel ratio has been attained.